In the fabrication of data-storage media (both magnetic-recording and magneto-optical), the most common technique for depositing the various thin-film layers is magnetron diode sputtering. The sputtering system approaches that are used in the production of magnetic-recording media consist mainly of two configurations: 1) for each thin-film layer, dual-side coating of a single disk substrate in the static-deposition mode from a pair of circularly symmetric planar (or planar ring) magnetron diode sputtering sources, and 2) for each thin-film layer, dual-side coating of a platen of several disk substrates in the dynamic-deposition (passby) mode from a pair of rectangular planar magnetron diode sputtering sources.
Both approaches are best accomplished in systems where process-isolated chambers are provided so that the substrate outgassing, sputter-etch cleaning, and heating steps, from which reactive gases (water vapor, air, organic solvents) are evolved, do not interfere with the inert-gas sputter-deposition steps. Similarly, a sputter-deposition step, involving a target material where a reactive gas (or gases) is intentionally employed or involving a target material whose composition comprises a reactive-gas constituent or whose microstructure contains a reactive gas (or gases) in its micropores, must be carried out in an isolated process chamber. This extremely important aspect pertaining to vacuum cleanliness is much more easily implemented in a system design based upon static deposition than on one based upon passby deposition. The presence of reactive gases during sputtering has a deleterious effect on the morphology and the magnetic properties of the deposited films, greatly affecting their uniformity and reproducibility.
In the aforementioned static- and dynamic-deposition sputtering system approaches, due to the requirement of product throughput, the sequential steps are essentially simultaneously operated so that there is a disk at or a platen of disks passing by each of the several stations, respectively. The static-deposition approach in which a single disk per pair of circular symmetric sputtering sources is employed has a significant advantage in that excellent circumferential uniformity of film morphology and thus of magnetic characteristics is obtained, even at the lowest (which is highly desirable) sputtering gas pressure. However, there are disadvantages to this particular static-deposition approach in that the product throughput and coating cost are independent of disk diameter and the economies of scale are not available for costly components, such as source power supplies and chamber vacuum valves, pumps, gauges, mass flow controllers, the transport mechanism, the process sequencer, etc.--the major cost-determining items of the sputtering system.
This situation is reversed in the passby approach where a platen of several disks per pair of rectangular line-deposition sputtering sources is employed. However, the serious disadvantage entailed in the passby approach is the lack of circumferential uniformity of film morphology and hence of magnetic characteristics along the circular tracks of the disk. These undesirable features are caused by the source-determined differing and changing angles of incidence of the arriving sputtered atoms at the substrate as the platen passes by the line-deposition sources. This problem can be overcome by operation at higher sputtering gas pressure or with increased source-to-substrate separation or with some combination of both. Since the sputtered atoms suffer because many more gas collisions occur in transit, as their directionality is lost and their arrival angles become randomized, the advantages of low-pressure deposition are thereby sacrificed. The once energetic sputtered atoms, losing their energy by gas collisional scattering, become thermalized. Consequently, the adhesive strength of the film to the substrate decreases, with an abrupt interfacial boundary forming instead of a graded diffused one. Additionally, the cohesive strength of the film decreases, with the thermalized-atom-deposited porous columnar Zone 1 or the still more porous Zone 1' (on the Movchan-Demchishin-Thornton zone-structure diagram) structure resulting instead of the energetic-atom-deposited dense fibrous Zone T structure.
Another sputtering system approach, though not in general use for the production of data-storage media, entails the use of high-radio frequency-powered (13.56 MHz) large-area circularly symmetric planar disk diode sputtering sources. In this approach for each thin-film layer, dual-sources. side coating of a platen of several disk substrates in the static-deposition mode from a pair of these large-area sputtering sources would be employed, thereby combining the advantages of the two aforementioned approaches with none of their respective disadvantages. There is, however, one inherent disadvantage of the 13.56 MHz rf planar disk diode sputtering source; namely, for a given system configuration, moderate deposition rates are obtained instead of the high deposition rates obtained with magnetron diode sputtering sources. Furthermore, in rf planar disk diode sources the sputtering-target utilization is greater than 90%, whereas in planar magnetron diode sources it is typically less than 30%, particularly for target materials of ferromagnetic and ferrimagnetic substances.
In addition, a properly designed sputtering system configuration with two like oppositely facing rf planar disk diode sources requires that the chamber diameter or box size be at least three times the diameter of the source so that there be sufficient grounded area in contact with the gas discharge in order to keep the plasma potential low with respect to ground. Since such systems are geometry dependent with the rf power and hence voltage dividing according to the respective areas of the target electrode (or electrodes) and of the grounded walls in contact with the gas discharge (the Koenig-Maissel relationship), the plasma potential with respect to ground then (1) increases with increasing geometrical confinement for a given power input, (2) increases with increasing power input for a given pressure, and (3) decreases with increasing pressure for a given power input. Thus, inadvertently, grounded surfaces and insulating surfaces, too, become subject to energetic ion bombardment (i.e., sputtering) in systems with geometrically confined rf discharges, resulting in the contamination of the sputter-deposited films with the materials of the chamber. The significant references further describing this phenomenon are as follows: H.R. Koenig and L. I. Maissel, IBM J. Res. Develop. 14, 168 (1970); J. W. Coburn and E. Kay, J. Appl. Phys. 43, 4965 (1972); and J. L. Vossen, J. Electrochem. Soc. 126, 319 (1979).
The moderate-deposition-rate limitation and the large-grounded-area requirement of the rf planar disk diode sputtering source can be overcome by means of a supported gas discharge. The means by which this feat is accomplished is the subject of this invention.
A method and device for obtaining a large-volume, high-density, and homogeneous plasma have been described in the U.S. Patent, "Method and Device for Exciting a Plasma Using Microwaves at the Electronic Cyclotronic Resonance," issued to M. Pichot, J. Pelletier, and Y. Arnal, U.S. Pat. No. 4,745,337, issued May 17, 1988 (filed Jun. 6, 1986), assigned to Centre National d'Etudes des Telecommunications; Centre National De La Recherche Scientifique, both of Paris, France, and is herein incorporated by reference.
This method and device utilize a coaxial structure for introducing microwave power to a ring antenna array inside a multipolar magnetic confinement structure at the electron-cyclotron-resonance condition in order to sustain a gas discharge with a high plasma density and a low plasma potential. Within the central magnetic-field-free region, the plasma is homogeneous, isotropic, and uniform in electron/ion density. By such means a large useful area is provided for plasma processing to which may be added another structure.
In a magnetic disk file, the most common recording medium is a very flat and smooth aluminum-alloy disk having both its surfaces coated with a ferrimagnetic or ferro-magnetic oxide powder dispersed in a resin binder or a ferromagnetic cobalt alloy as a plated or sputter-deposited thin film. Aluminum is a soft metal and therefore, in the attempt to avoid read/write failure caused by the read/write head impacting the disk too forcefully, the aluminum-alloy disk is typically first coated with a hard material before the ferromagnetic cobalt alloy is applied. However, typically the surfaces coated with hard materials need to be either diamond tool turned on a lathe or ground by a surface grinder. Other labor-intensive methods are used to attempt to provide a surface free of nodules, pits and bumps; however, typically, these labor-intensive methods fall short of their intended purpose.
In the magneto-optical disk file, typically an active recording layer is applied to either a substrate or a substrate coating layer. The active recording layer is a ferromagnetic material with a substantial Kerr magneto-optical effect. When an opposing magnetic field is applied to this material locally laser heated to near but below the compensation or Curie temperature, the alignment of the magnetic moments may be reversed.
In the case where the ferromagnetic material is layered on or near a metal substrate disk, as is the case for magneto-optical recording disks, eddy currents are generated within the metal substrate by the applied magnetic field and affect the alignment of the magnetic moments of the ferromagnetic material. Instead of providing a preferred orderly alignment of magnetic moments, disorder is introduced into the system of the ferromagnetic material by the eddy currents. The disorder causes an undesirable reduced signal-to-noise ratio of the output signals.